Hey Mr. Green,
How can I calculate how much carbon I offset by composting, planting trees, recycling, growing vegetables, etc? –-Kerrey, in Houston
There are a number of Internet calculators for your virtuous activities, the most comprehensive being those from government sources such as the USDA and the EPA.
For trees, go to the USDA's Climate Change Resource Center, where you can download a calculator that will allow you find the carbon offset of trees, http://www.fs.fed.us/ccrc/topics/urban-forests/ctcc/
To determine the offset from recycling or composting of darn near anything recyclable or compostable, see The EPA’s Waste Reduction Model (WARM) http://www.epa.gov/climatechange/wycd/waste/calculators/Warm_Form.html. Since it is geared toward municipal operations that handle tons of material, you’ll have to adjust for your volume. There are tools in this EPA site that can help you refine your calculations. For example, if you know how far your local collectors would have to go to haul material to the dump instead of recycling, you can factor this in.
To see how much you can offset by purchasing goods that have a recycled content, see the EPA’s Recycled Content (ReCon) Tool, http://www.epa.gov/climatechange/wycd/waste/calculators/ReCon_Online.html
As for veggies, frankly you’d be way better off spending your time busting your buns in your garden instead of wasting hours calculating its offset potential. This is because computing the offset is hopelessly complicated, since different vegetables can have different emissions factors in growing and processing, different emissions factors in different soils and climates, different shipping distances and modes of transportation, and different irrigation requirements which demand different amounts of energy to pump water, etc., etc.
To illustrate: suppose you grow 100 pounds of potatoes in your garden, and you want to find out the offset from just one of a gazillion factors: transportation. First you have to determine how far the potato field is from your food store. After a half-dozen phone calls, you manage to find out that it’s 500 miles. Assuming the potatoes are shipped in a semi truck, further research reveals that the truck gets around 6 miles per gallon hauling a fairly typical 60,000 pounds. Next, you calculate that your potatoes amount to 100/60,000 = .00167 of the load. Then you figure that a 500-mile trip would require 500/6 = 83.3 gallons of diesel fuel. Next, using the numbers above, you calculate that your potatoes' share of diesel fuel moving that load is .00167 (83.3) = .139 gallons. Next, you look up the amount of carbon dioxide emitted by burning diesel fuel, which is about 22.3 pounds per gallon, and finally you multiply 22.3 by .139, and learn, if you've done the math to this point, that your 100 pounds of potatoes will offset 3.01 pounds of transportation-related carbon dioxide. And that’s just one factor: For a comprehensive analysis, you’d also have to consider your offsets from cropping, processing, storage packaging, and retailing.
So, delving deeper into such investigations and computations could spell agricultural disaster, where you're glued to a phone and a computer while weeds invade your garden and tater-planting time passes you by ! Sometimes it’s best just to do the right thing without waiting for mathematical justification.
What I’ve presented here just scratches the surface. For more insight into the complexities of all calculations, take a peek at the EPA's carbon inventory data at http://epa.gov/climatechange/emissions/downloads10/US-GHG-Inventory-2010_ExecutiveSummary.pdf